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Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Schedule of Equity Method Investments [Line Items]        
Net investment income earned $ 45,553 $ 41,430 $ 88,784 $ 78,849
Alternative Investments [Member]        
Schedule of Equity Method Investments [Line Items]        
Net investment income earned 1,900 5,200 3,500 6,800
Investments accounted for under the equity method [Member]        
Schedule of Equity Method Investments [Line Items]        
Net investment loss (6,400) (88,000) (41,800) (62,400)
Realized gains (losses) 629,500 (69,200) 1,223,500 (304,300)
Net change in unrealized (depreciation) appreciation (1,200,200) 1,328,500 (738,600) 1,890,000
Net (loss) gain $ (577,100) $ 1,171,300 $ 443,100 $ 1,523,300